Active matrix pixel device

ABSTRACT

A method of constructing an active matrix pixel device uses a universal active matrix (UAM) ( 81 ) comprising a matrix array of switching elements ( 20 ) (whose spacing defines a base pitch and a pixel array comprising a matric array of pixel electrodes ( 19 ) whose spacing defines a pixel pitch. The pixel pitch is greater than the base pitch. A large proportion of the construction process can be carried out before the customisation of the device. Using a common UAM ( 81 ) enables a reduction in the time between the customer ordering the device and the completion time. The cost of meeting customer specific requirements for the fabrication of active matrix pixel devices is thus reduced.

[0001] This invention relates to active matrix pixel devices and theirmethod of fabrication, and more particularly, but not exclusively, tothe customisation of active matrix displays, and especially activematrix liquid crystal displays (AMLCDs), where a range of pixel pitchescan be achieved from an initial universal active matrix.

[0002] Examples of active matrix pixel devices other than displaydevices include sensing devices such as image sensing devices andfingerprint sensing devices in which the matrix elements comprise forexample optical or capacitance sensing elements, transducer devices, inwhich the matrix elements comprise moveable electromechanical elements,for example piezoelectric or electrostatically controlled actuatorelements.

[0003] Active matrix pixel devices, such as AMLCDs, are used in anincreasingly wide variety of products, including consumer electronics,computers and communication devices. Such devices are often included inportable products where the size and compactness of the product areparticularly important considerations.

[0004] An example of such a device is described in EP-A-0617310. In thisdevice, a row and column matrix array of display pixels is provided,each of which is driven via an associated switching element in the formof a TFT (thin film transistor). As is usual, the device comprises alayer of liquid crystal (LC) material disposed between a pair of spacedsubstrates carrying electrodes which define individual display pixels.The TFTs are carried on the surface of a first substrate together withsets of row, (scanning), conductors and column, (data), conductorsthrough which the TFTs are addressed for driving the display pixels.Each TFT is disposed adjacent the intersection between respective onesof the row and column conductors. The gates of all the TFTs associatedwith a row of display pixels are connected to a respective row conductorand the sources of all the TFTs associated with a column of pixels areconnected to a respective column conductor. This forms an array ofactive cells in which each cell comprises a TFT having associated rowand column conductors. An array of reflective metal pixel electrodes iscarried on an insulating film which extends over the first substrate andcovers the TFTs and the sets of address conductors so that the pixelelectrodes are positioned generally above the level of the TFTs and theaddress conductors. As is conventional, each pixel electrode isassociated with one respective TFT. Each individual pixel electrode isconnected to an underlying contact electrode, which is integral with thedrain electrode of its associated TFT, through a respective openingformed in the insulating film directly over the contact electrode. Withthis type of construction, in which the array of pixel electrodes andthe array of TFTs are provided at respective different levels above thesubstrate surface, the pixel electrodes can be enlarged such that at twoopposing sides they extend slightly over adjacent row conductors and attheir two other opposing sides they extend slightly over adjacent columnconductors, rather than being sized smaller than the spacing betweenadjacent row conductors and adjacent column conductors with gapsprovided between each edge of the pixel electrode and the adjacentconductor, as in display device arrangements in which the individualpixel electrodes are arranged substantially co-planar with, andlaterally of, the TFTs. In this way, therefore, the pixel aperture isincreased and in operation more light which passes through the LC layerand reaches the pixel electrode is reflected back to produce a brighterdisplay output. Moreover, parts of a deposited metal layer which ispatterned to form the reflective pixel electrodes can be leftimmediately overlying the TFTs during the patterning process so as toact as light shields for the TFTs to reduce photoelectric effects in theTFTs due to light incident thereon, thereby avoiding the need to provideblack matrix material on the other substrate for this purpose. Thesubstrate carrying the TFTs, address conductors and pixel electrodesconstitutes the active plate of the display device. The other,transparent, substrate constitutes the passive plate and carries acontinuous transparent electrode common to all pixels in the array andan array of colour filter elements corresponding to the array of pixelswith each filter element overlying a respective pixel electrode.

[0005] In order to supplement the capacitance of each LC cell in anAMLCD, a storage capacitance is commonly provided in parallel with theLC cell. This is required to maintain the desired voltage across thecell when the driving signal is removed. One example method of providinga storage capacitor is to form an extra conductive layer over thecontact electrode with a dielectric material sandwiched in between.

[0006] Conventionally, the active matrix array is fabricated bydepositing on the substrate various layers of conductive, insulating andsemiconductive layers and patterning these layers using aphotolithographic definition process involving photolithographic masksthat determine the pattern of individual layers.

[0007] There is growing interest in making AMLCDs which can be easily,quickly and cheaply customised to the needs of a particular application.For example, different customers may require displays having differentand specific pixel pitches. This is especially true for small/mid-sizeddisplays for applications in portable products, such as mobile phones,PDAs, and the like, where the market is characterised by constant changein product ranges and hence display designs.

[0008] It is well known in the art that to produce a batch of displaysto a new design requires a complete new set of photolithography masks. Aproblem is that this can make customised AMLCDs rather expensive, as theinvestment costs in a mask set are high and the total number of displaysover which these costs can be recovered may be quite small. In addition,lead times can be long as masks must be designed for each specificcustomer requirement and each customer specific product must then beprocessed through to completion.

[0009] It is an object of the present invention to provide an improvedmethod of producing an active matrix pixel device.

[0010] It is another object of the present invention to provide a methodof producing an active matrix pixel device allowing for a cost reductionin meeting customer specific requirements.

[0011] According to one aspect of the present invention there isprovided a method of constructing an active matrix pixel devicecomprising:

[0012] providing a universal active matrix comprising on a substrate amatrix array of switching elements whose spacing defines a base pitchand sets of row address conductors and column address conductors foraddressing the switching elements;

[0013] forming on the substrate a dielectric layer over the array ofswitching elements,

[0014] forming an array of contact holes in the dielectric layer suchthat contact can be made with a plurality of switching elements,

[0015] forming a pixel array on the universal active matrix, the pixelarray comprising a matrix array of pixel electrodes in electricalcontact with underlying switching elements via the contact holes, thespacing of the pixel electrodes defining a pixel pitch,

[0016] wherein the pixel pitch is greater than the base pitch.

[0017] When the universal active matrix (UAM) has been formed on thefirst substrate, the partly constructed device can be stockpiled ifrequired. The pixel array with a desired pixel pitch can then be formedat a later stage to meet the customers' requirements. One advantage thatthe invention provides is that a large proportion of the productionprocess can be carried out before the customisation. Such a methodenables active matrix pixel devices of differing designs, e.g. havingdifferent pixel electrode layouts, to be fabricated more readily, andless expensively, than previously. Using a common UAM, which can bestockpiled for convenience, enables a reduction in the time between thecustomer ordering the device and the completion time. Another advantageis that less mask sets are required for each new custom active matrixpixel device as the same masks can be used for each UAM. This reducesthe cost of customised devices.

[0018] According to another aspect of the invention there is provided anactive matrix pixel device comprising a universal active matrix having amatrix array of switching elements arranged so as to define a basepitch, and a pixel layer having a matrix array of pixel electrodesarranged so as to define a pixel pitch, wherein the pixel pitch isgreater than the base pitch.

[0019] The switching elements of the UAM are preferably addressed byrespective row and column address conductors as described previously.Each switching element with its respective row and column addressconductors forms an active cell. The spacing of the active cells definesthe base pitch. The spacing may not necessarily be the same in thehorizontal (row) and vertical (column) directions and so the base pitchcan be sub-divided into horizontal and vertical components if necessary.The base pitch is preferably made as small as is conveniently possible.This defines the minimum achievable pitch of the final active matrixpixel device. By minimising the base pitch in this way the range ofachievable pixel pitches for the final product is increased.

[0020] In a preferred embodiment a dielectric layer overlies the arrayof switching elements, preferably comprising a polymer material andcovering the whole array. This acts to reduce significantly capacitivecoupling between the overlying pixel electrode layer and the underlyingUAM.

[0021] The switching elements preferably comprise thin film transistors(TFTs) as known in the art. The TFTs may be top-gate or bottom-gate typeTFTs. TFTs in matrix arrays are normally fabricated as respective,separate, semiconductor islands of amorphous, polycrystalline ormicrocrystalline silicon material or a plastics organic material definedby patterning a continuous semiconductor layer deposited over thesubstrate to leave discrete areas of semiconductor material arranged ina row and column matrix.

[0022] As the pixel pitch is greater than the base pitch, some pixelelectrodes may cover more than one switching element. Electrical contactcan be made with at least one of the covered switching elements via thecontact holes in the dielectric layer. In a preferred embodiment of thepresent invention each pixel electrode is connected to and controlled byjust one of the underlying switching elements. Therefore some switchingelements are left redundant and are not used to supply signals to thepixel electrodes. An advantage of using only one switching element perpixel electrode is that there is less capacitive coupling between therow and column address conductors. This helps to reduce displayartefacts such as cross-talk and flicker.

[0023] In another embodiment individual pixel electrodes may beaddressed simultaneously by more than one underlying switching element.This can be done by connecting the associated adjacent row and columnconductors together in parallel.

[0024] Although the base pitch and pixel pitch are substantiallyunrelated it is envisaged that there may be an integral relationship inwhich the pixel pitch is an integer multiple of the base pitch in boththe horizontal and vertical components thereof. For example, where thehorizontal pixel pitch is three times the horizontal base pitch and thevertical pixel pitch twice the vertical base pitch, each pixel electrodecovers six switching elements. Therefore any one or more of theseswitching elements can be utilised if necessary to operate theirassociated overlying pixel electrode. According to a further aspect ofthe present invention the active matrix pixel device comprises an AMLCDdevice. In preferred embodiments the AMLCD device comprises a reflectiveor transflective type of display device.

[0025] Although the active matrix pixel device according to the presentinvention preferably comprises a liquid crystal display it is envisagedthat the invention can be applied to other types of active matrixdisplay devices, for example electrophoretic, electrochromic orelectroluminescent display devices, and also to active matrix pixeldevices for non-display purposes, for example sensor arrays such asimage sensing arrays, etc.

[0026] Embodiments of active matrix pixel devices and their method offabrication in accordance with the invention will now be described, byway of example, with reference to the accompanying drawings, in which:

[0027]FIG. 1 is a plan schematic view of part of an active matrix pixeldevice fabricated by a first embodiment of the method in accordance withthe present invention;

[0028]FIG. 2 is a plan schematic view of part of an active matrix pixeldevice fabricated by a second embodiment of the method in accordancewith the present invention illustrating the structure of a typicalactive cell;

[0029]FIGS. 3A to 3G illustrate schematically in section various stagesin the method of fabricating the device of FIG. 2;

[0030]FIG. 4 is a flow-chart describing the method of construction asillustrated in FIGS. 3A to 3G;

[0031]FIG. 5 is a plan schematic view of part of a device fabricated bya third embodiment of the method according to the present invention;

[0032]FIG. 6 illustrates one preferred arrangement of part of the deviceduring its fabrication;

[0033]FIG. 7 illustrates a second preferred arrangement of part of thedevice during its fabrication;

[0034]FIG. 8 shows schematically the structure of a complete AMLCD.

[0035] It should be understood that the Figures are merely schematic andare not drawn to scale. In particular, certain dimensions may have beenexaggerated whilst others have been reduced. The same reference numeralsare used throughout the drawings to indicate the same or similar parts.

[0036]FIG. 1 illustrates schematically part of a device fabricated by afirst embodiment of the method in accordance with the present invention.The device comprises a reflective type active matrix liquid crystaldisplay (AMLCD) panel 11 having an array of pixel electrodes overlying auniversal active matrix (UAM). It will be appreciated that FIG. 1 showsa small part of the AMLCD with only one complete pixel electrode andthat a typical device will comprise an array having hundreds of pixelelectrodes, e.g. around 150×150 in the case of mobile phone displays.The UAM comprises on a substrate a circuit comprising a set of regularlyspaced column address conductors 12 extending parallel to one another inthe vertical direction and a set of regularly spaced row addressconductors 14 extending parallel to one another in the horizontaldirection. A switching element 20 comprising, in this example, a bottomgate type thin film transistor (TFT), is formed at, or near, eachintersection of the row and column conductors. Each row conductor 14provides the gate electrodes 15 for each TFT in a respective row. Alayer of semiconductor material, not shown, overlies the gate electrodeof each TFT. Each column conductor 12 provides the source electrodes 13for each TFT in a respective column. Each source electrode overlapspartially with the TFT's gate electrode. Each TFT also comprises a drainelectrode 17 formed from a layer of conductive material which drainelectrode partially overlaps with each respective gate electrode 15 andwhich layer extends partially over the active cell defined by theadjacent row and column conductors forming an integral contact electrode16.

[0037] The UAM comprising this array of TFTs and associated addressingconductors is common to all devices according to the invention. The TFTsare regularly spaced and define a base pitch. The base pitch is notnecessarily the same in the horizontal and vertical directions. Elementsof the display device using the part shown in FIG. 1, which will bedescribed hereinafter, can be varied so as to customise the pixel pitchof the final product.

[0038] A dielectric layer, not shown in FIG. 1, completely overlies thearray of active cells. An array of similarly sized and regularly spacedpixel electrodes 19 overlies the dielectric layer. Each pixel electrode19 makes contact with at least one underlying contact electrode 16 via acontact hole 36 in the dielectric layer. The number, location and pitchof the contact holes 36 is dependant on the required pixel pitch of thefinal product. The embodiment of FIG. 1 shows part of a display devicewith a horizontal pixel pitch of just over one and a half times that ofthe horizontal UAM pitch. The vertical pixel pitch is about three timesthat of the vertical UAM pitch. The complete pixel electrode 19 showncovers two complete active cells including their respective TFTs andcontact electrodes 16. This pixel electrode 19 makes contact with onecontact electrode 16 via a contact hole 36. Therefore this pixel isaddressed by only the row and column address conductor associated withthat TFT. Similarly, each of the other pixel electrodes 19 in the arrayof pixel electrodes makes contact with one underlying contact electrodevia a contact hole 36.

[0039] Although in the preferred embodiment of FIG. 1 each pixelelectrode 19 only makes contact with one contact electrode 16 of one ofthe underlying TFTs, it is envisaged that contact can instead be madewith a plurality of underlying contact electrodes such that each pixelcan be addressed by more than one TFT. In this case, the row and columnconductors associated with each of the TFTs would need to be addressed.This can be accomplished conveniently by interconnecting the row andcolumn conductors respectively at their ends so that they receive thesame drive (scanning and data) signals simultaneously.

[0040]FIG. 1 shows TFTs of the bottom-gate type where the gate electrodeunderlies the semiconductor material. Devices made in accordance withthe present invention may instead comprise top-gate TFTs where the gateelectrode overlies the semiconductor material.

[0041]FIG. 2 is a schematic view of part of an active matrix pixeldevice fabricated by a second embodiment of the method in accordancewith the present invention and shows an active cell of the UAM havingone switching element 20 comprising a top-gate TFT, and a storagecapacitor. The various stages of fabrication of the device of FIG. 2 areset out in FIGS. 3A to 3G which show a cross-section along the line A-Aof FIG. 2. FIG. 4 is a process flow-chart summarising the separatestages of construction of a device having a base pitch which differsfrom the pixel pitch.

[0042] With reference to FIG. 3A, a first conductive layer 32,preferably transparent, e.g. of Indium Tin Oxide (ITO), is depositedcompletely over an insulating substrate 31, preferably glass. A secondconductive layer 21, preferably MoCr, is deposited on the firstconductive layer 32. The second layer 21 is etched using aphotolithographic patterning process to form an integral part of thecolumn conductor 12. The material of the second conductive layerpreferably has a higher conductivity than that of the first conductivelayer.

[0043] The first conductive layer 32 is then etched, as shown in FIG.3B, so as to form the column conductors 12 with the source electrodes 13for the TFTs and the contact electrodes 16 integral with the drainelectrodes of the respective TFT. The contact electrodes 16 form thefirst plate for a storage capacitor.

[0044] With reference to FIG. 3C, a semiconductor layer 22, preferablyof amorphous-silicon, is then deposited and patterned so as to providethe active layer of the TFT. A first insulating layer, preferably ofsilicon-nitride (1), is deposited and patterned over the columnconductors 12 to define selected insulator regions 23. These insulatethe column conductors from the row conductors and other crossingfeatures at their cross-over regions, which are added at a later stage.

[0045] A second insulating layer 33, preferably of silicon-nitride (2),is deposited over the substrate. A respective contact hole 18 is formedthrough this layer over each contact electrode 16, as shown in FIG. 3D.

[0046] With reference to FIG. 3E, a metal layer, for example AlCu, isthen deposited over the substrate. This is patternedphotolithographically and etched to form the row conductors 14 with agate electrode 15 for each TFT overlapping the semiconductor layer 22.The second storage capacitor plate 24 and respective capacitor conductorlines 25, extending parallel with the row conductors, for the storagecapacitors are also patterned from this deposition. The insulating layer33 therefore provides the intervening dielectric for the storagecapacitors. This completes the construction of the UAM.

[0047] At this stage of the fabrication process the UAM is stockpiledfor use in customised display panels. This UAM can then be used toproduce a variety of active matrix display panels of this type,irrespective of their required pixel pitch. Only one mask set isrequired to produce the UAM and mask-sets that would otherwise berequired for producing devices with other specific pixel pitches, up tothis stage of production, are not required, therefore saving cost.

[0048] Once the required pixel pitch is known, the device can becustomised as described hereinafter. With reference to FIG. 3F, adielectric layer 34 of silicon-nitride (2) is deposited completely overthe UAM. Contact holes 36 are formed in this at the required locations.The locations of the contact holes 36 correspond to selected ones of thecontact holes 18 and are dependent on the required pixel pitch and thenumber of TFTs that are required for driving each pixel electrode 19.The dielectric layer 34 is also patterned to allow contact with the rowand column conductors, 14 and 12, at the edge of the device forconnection to the outputs of respective driver circuits (not shown).

[0049] A further conductive layer is then deposited and patterned toform the reflective pixel electrodes 19 as shown in FIG. 3G. Each pixelelectrode contacts at least one contact electrode 16 via a contact hole36. The further conductive layer may be a metal or ITO on a silver or asilver alloy layer.

[0050] As can be appreciated, the size of the individual pixelelectrodes 19 and their pitch can be varied as described for thecustomisation of the device.

[0051]FIG. 4, showing a flow-chart of the above described method ofconstruction, illustrates that only two steps in the fabrication processare customised to meet the requirements of the desired pixel electrodepitch. This completes the fabrication of the active plate of the activematrix pixel device.

[0052]FIG. 5 shows part of an active matrix pixel device fabricated by athird embodiment of the method according to the present invention. Thedevice comprises row and column conductors, 14 and 12, as describedhereinbefore with reference to the previous embodiments. However, eachpixel electrode 19 is connected to a plurality of contact electrodes 16via contact holes 36 and each pixel electrode is therefore controlled bymore than one TFT. From the figure it will be appreciated that there isa non-integral relationship between the base pitch and the pixel pitch.The horizontal pixel pitch is a little under two and a half times thatof the horizontal base pitch. The vertical pixel pitch is a little underthree and a half times that of the vertical base pitch. Each pixelelectrode 19 in a device with a pitch relationship of this kind willoverlie at least six complete active cells and therefore sixcorresponding TFTs. The adjacent column conductors 12 that address eachcolumn of cells that a pixel electrode overlies are connected togetherby a shorting bar 51. Each column of TFTs associated with each shortingbar 51 are therefore addressed in parallel. The adjacent row conductors14 associated with cells that a pixel electrode overlies are connectedtogether with a similar shorting bar 53. The rows of TFTs associatedwith each shorting bar 53 are therefore each selected in parallel. Dueto the non-integral relationship between the base pitch and pixel pitch,there are redundant column conductors 52 and redundant row conductors54. These may be shorted together to keep all redundant row or columnconductors at a fixed potential if required. Cells which underlie twoadjacent pixel electrodes are not provided with contact holes 36.

[0053]FIGS. 6 and 7 show alternative layouts for the TFT electrodes ofthe UAM. They both employ a bottom gate TFT similar to that of theembodiment illustrated in FIG. 1. The column conductors 12, rowconductors 14 and capacitor conductor lines 25 are insulated from oneanother by an intervening layer of insulating material 62. Asemi-conducting layer 22 forms the active layer of each TFT and contactswith the source, drain and gate electrodes. In this arrangement astorage capacitor is provided at an overlap region between the contactelectrode 16 and the capacitor conductor line 25. The insulatingmaterial 62 forms the dielectric for the storage capacitor for each cellby separating the corresponding capacitor conductor 25 from an area ofthe contact electrode 16.

[0054] To maximise the achievable range of pixel pitches for the finalproduct, it is desirable to have a relatively small base pitch. FIG. 6shows one possible arrangement where each row of TFTs has acorresponding capacitor conductor line 25 and each column of TFTs has acorresponding column conductor 12. In order to minimise the base pitch,special layouts can be utilised as illustrated in FIG. 7. Here, eachcolumn of TFTs corresponds to two column conductors 12. Pairs ofadjacent TFT rows share one storage capacitor conductor line 25. Thismakes better use of the array space. Under normal conditions, such anarrangement would lead to undesirable image artefacts as the localenvironment of different rows and columns differ. However, where theindividual pixels are driven by more than one TFT, these effects will bereduced as some averaging will take place over the area of theindividual pixels.

[0055] The manufacturing process may comprise the further step (notshown) of depositing a Diffusing Organic Layer (DOL) over the UAM afterforming the TFT switching elements and the storage capacitors asdescribed with reference to FIGS. 2, 3 and 4. This comprises a layer ofbumps which, after the customised layers are deposited, results incorresponding bumps being formed in the pixel electrodes. In thecompleted display device, these pixel electrode bumps serve to diffusethe reflected light so as to ensure non-specular reflection when viewed.This type of layer is described in EP-A-0617310. An alternative methodof ensuring non-specular reflection is to incorporate a front scatteringfilm on the passive plate of the display panel.

[0056]FIG. 8 shows the structure of a complete liquid crystal displaywhich incorporates the active plate comprising the UAM, here referencedgenerally at 81, and the customised pixel electrode layer 82. A layer ofliquid crystal material 80 is provided over the pixel electrode layer82, which comprises the structure described above. The second, passive,substrate, here shown at 83, overlies the layer of liquid crystalmaterial. This further substrate 83 may be provided on one face with anarrangement of colour filters 84 and a layer defining the commonelectrode 87. A polarising film 86 is also provided on the substrate 83.

[0057] As this invention is concerned specifically with the active,transistor, substrate and the reflective pixel electrodes, the operationand construction of the liquid crystal display will not be described inany further detail as this will be apparent to those skilled in the art.

[0058] Additional layers to those described may be provided, and thereare various alternatives which will be apparent to those skilled in theart. The specific processing parameters and materials have not beendescribed in detail in this application, as this invention relies uponknown individual processing steps and materials. The steps, and therange of possible alternatives, will be apparent to those skilled in theart.

[0059] The specific example above uses amorphous silicon TFTs in the UAMof the LCD, but other semiconductor materials are possible, such aspolycrystalline or microcrystalline silicon.

[0060] From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in the field of active matrixpixel devices and component parts therefor and which may be used insteadof or in addition to features already described herein.

1. A method of constructing an active matrix pixel device comprising:providing a universal active matrix comprising on a substrate a matrixarray of switching elements whose spacing defines a base pitch and setsof row address conductors and column address conductors for addressingthe switching elements; forming on the substrate a dielectric layer overthe array of switching elements, forming an array of contact holes inthe dielectric layer such that contact can be made with a plurality ofswitching elements, forming a pixel array on the universal activematrix, the pixel array comprising a matrix array of pixel electrodes inelectrical contact with underlying switching elements via the contactholes, the spacing of the pixel electrodes defining a pixel pitch,wherein the pixel pitch is greater than the base pitch.
 2. The method ofclaim 1, wherein the array of contact holes is formed such that only aselected proportion of the switching elements are connected to pixelelectrodes.
 3. The method of claims 1 or 2, wherein the array of contactholes is formed such that at least some of the pixel electrodes are eachin electrical contact with only one switching element.
 4. The method ofclaims 1, 2 or 3, wherein the pixel array is formed such that the pixelpitch is an integer multiple of the base pitch.
 5. An active matrixpixel device comprising: a universal active matrix comprising a matrixarray of switching elements whose spacing defines a base pitch; and, apixel array comprising a matrix array of pixel electrodes whose spacingdefines a pixel pitch, wherein the pixel pitch is greater than the basepitch.
 6. The device of claim 5, wherein a dielectric layer separatesthe universal active matrix from the pixel array.
 7. The device of claim6, wherein the dielectric layer comprises contact holes to allow eachpixel electrode to contact with at least one underlying switchingelement.
 8. The device of claims 5, 6 or 7, wherein only a proportion ofthe switching elements are connected to pixel electrodes.
 9. The deviceof any of claims 5 to 8, wherein at least some of the pixel electrodesare each in electrical contact with only one switching element.
 10. Thedevice of any of claims 5 to 9, wherein the pixel pitch is an integermultiple of the base pitch.
 11. A liquid crystal display devicecomprising an active matrix pixel device of any of claims 5 to
 10. 12.The display device of claim 11, wherein the display is a reflective ortransflective display device.
 13. The device of any of claims 5 to 12,wherein the switching elements comprise thin film transistors.